Study of the conductivity behavior of La0.75Ca0.25MnO3 in the vicinity of the ferromagnetic-paramagnetic transition with the use of single current pulses

The temperature and current dependences of the resistance of a La_0.75Ca_0.25MnO₃ sample have been investigated using single ramp current pulses. It is found that the sample resistance depends strongly on the current amplitude near the Curie temperature T _C. The observed features can be related to nonuniform Joule heating caused by the mixed phase state of manganites near the ferromagnetic-paramagnetic transition and the percolation character of this transition.     

Long-term stability of Bi(2223) and Dy(123) superconducting tapes in the direct current circuit

The stability of critical parameters T _c and I _c of commercial high-temperature superconducting wires upon long-term passage of transport current (about 0.7I _c) in liquid nitrogen (77 K) is studied. Voltage-current characteristics U(I), as well as the critical current and critical temperature, are investigated for the case of Bi(2223) hermetic multifilament wires and Dy(123) superconducting tapes covered by a thin Ag layer. In the former case, a considerable decrease in the critical current (by ～30%) and swelling of the wires after passage of the current for 323 h are observed. The same is true for a reference sample, which does not experience the action of current and stays in liquid nitrogen for 700 h. The decrease in the critical current in the Bi(2223) sample is likely to be associated with penetration of a liquid coolant into the composite conductor: evaporating and expanding as a result of heating, it severely deforms the material. The Dy(123) sample grown epitaxially demonstrates high stability of the critical current after it has experienced the action of current for 400 h and been kept in liquid nitrogen for 1000 h.     

The current-induced effect on the Jahn-Teller distortion in the La0.5Ca0.5MnO3 manganite

The influence of dc current on the resistivity ρ and the Young's modulus E of La_0.5Ca_0.5MnO₃ compound has been investigated by means of an in situ measuring method. At low temperatures, both the resistivity ρ and the relative modulus ΔE increase with the current. A relaxation behavior of ρ to the higher resistive state is observed at a fixed temperature and a constant current. After storing the sample for a few days, ρ decreases with the current, accompanying a slight drop of ΔE at low temperatures. Current-induced effects on ρ and ΔE are interpreted according to the current-induced interwinning of Mn³⁺O₆ octahedral distortion modes between Q₂- and Q₃-types, which is suggested to contribute to the variation of the resistivity.     

New experimental criterion of the Berezinskii-Kosterlitz-Thouless transition

Based on the experimental study of YBa₂Cu₃O_7?x single crystal nonlinearity near T _c , a new criterion for observing the Berezinskii-Kosterlitz-Thouless (BKT) transition in cuprate HTSC compounds was proposed. The criterion consists in the existence of the maximum in the dependence of the voltage of the second harmonic of the sample response signal excited by alternating current on the direct bias current. It was shown that the position of the second harmonic maximum is independent of temperatures near T _c , which is consistent with the BKT transition independence of the measuring current.     

Effect of distribution of powder size on estimation of critical current densities in polycrystalline Sr0.6K0.4Fe2As2 superconductors

The effect of distribution of powder size on the estimation of intra-grain critical current density was investigated for polycrystalline samples of Sr_0.6K_0.4Fe₂As₂ with and without addition of Ag 20 wt.%. It turned out that the value of intra-grain critical current density calculated using the average of powder size J_cl is more overestimated than that calculated taking account of the distribution of powder size J_clP as the distribution of powder size is widened, since the magnetic moment drastically increases due to the larger contribution from larger particles. From the distribution of powder size obtained by the microscopic observation using SEM, the factor of overestimation of J_cl/J_clP is found to be larger than two in both samples. Moreover, the distribution of powder size in the Ag-added sample is found to be smaller than that of pure sample. Therefore, the factor of overestimation of J_cl is considered to be smaller in the Ag-added sample. It is recommended to take account of the distribution of powder size for precise estimation of intra-grain critical current density.     

Current induced deformation of charge density waves in orthorhombic TaS3

The low electric field ohmic resistance R of orthorhombic TaS₃ measured at 90 and 120 K well below the Peierls transition temperature depends on the product of a temperature difference ΔT applied along the sample and the sign of a previously applied current pulse if this pulse is larger than threshold for non-ohmic conductivity. This resistance change is about ΔR/RΔT ∽ 1×10^-3 K^-1 for a pure sample and ΔR/RΔT ∽ 6×10^-3 K^-1 for a slightly electron irradiated one at 90 K. The relative resistance change is insensitive to the sample length. We deduce that the CDW current changes inhomogeneously the Peierls gap E_g. ΔE_g < O at the contact where the CDW current enters and ΔE_g > O at the exit. The effect is attributed to a CDW current induced inhomogeneous deformation of the CDW itself.     

Magneto-Impedance behavior of Co-Fe-Nb-Si-B-based ribbons

The giant magneto-impedance of melt spun Co_xFe_72−xNb₄Si₄B₂₀(x=10, 20, 36, 50) amorphous and nanostructured ribbons have been investigated. Alloys have been optimized at the driving current amplitude, frequency and found that amorphous ribbon of nominal composition of Co₃₆Fe₃₆Nb₄Si₄B₂₀ shown maximum GMI ratio of 13%. The behaviour of the driving current amplitude on the GMI behaviour was studied and the sample was optimized for driving current amplitude, I_ac=10 mA. The frequency dependence of the GMI behaviour was studied for the ribbon sample Co₃₆Fe₃₆Nb₄Si₄B₂₀ at frequency in the range of 100 kHz-1.2 MHz of the optimized driving current amplitude and it was found that the sample showed the maximum GMI behaviour at f=700 kHz. The optimized samples were Joule heated at the current density J=0-35 A/m² for a period of 1 min. The GMI ratio initially increased then progressively deteriorated with J, but after a certain range it shows up to 16% of improvement in the magneto-impedance value due the increase of nanocrystalline volume fraction. The asymmetry in the GMI profile was observed for the sample Joule heated at J=1-5 A/m² for 1 min.     

Temperature dependence of electroresistance for La0.67Ba0.33MnO3 manganite

The influence of dc biasing current on temperature dependence of resistance of La_0.67Ba_0.33MnO₃ bulk sample is reported. A decrease in the resistance (electroresistance) on the application of higher bias current is observed. The electroresistance is maximum at metal-insulator transition temperature (T_MI) and decreases when the temperature is either increased or decreased from T_MI. A two-phase model is proposed to explain the occurrence of electroresistance. The higher bias current leads to an increase in alignment of spins and thus, in turn, leads to an increase in spin stiffness coefficient and decrease in the resistance at T_MI.     

Prediction of a domain-drag effect in uniaxial,non-compensated,ferromagnetic metals

An electric current is predicted to exert a dragging force on ferromagnetic domain walls. The effect arises from the non-uniform current distribution, recently predicted and observed at 4·2 K in the neighborhood of a wall in metals (Co, Ni) with large Hall angle. When the average current density j? exceeds a certain “coercive” value j?_c, the domains are set into motion in the same direction as the charge carriers. When j?j?_c, the wall speed approaches the drift speed of the carriers. Ohm's law fails when the walls move. The limits of very small and of very large Hall angle are both considered, using the single-wall model of Williams, Shockley and Kittel, and also the stratified-medium model of Herring. We assume B = M_s inside each domain; this is appropriate if the sample thickness t in the direction of the saturation magnetization M_s, and the wall spacing a, obey t > a. Materials with large anisotropy field H_A (h.c.p. Co, Gd) are necessary if the field H of the current is not to remove all walls normal to the current. For similar reasons, the sample should be in the shape of a flat ribbon (or rod) normal to M_s, of thickness (or diameter) t obeying t < dH_A/H_c, where H_c is the coercive field and d the wall length in the direction normal to the current and to M_s.     

Determination of the α parameter for the decay Ξ− → Λπ−

The α parameter for Ξ^? → Λπ^? decay has been determined to be α_Ξ^?=?0.462 ± 0.015, from a sample of 1.5 × 10⁵ events recorded in the CERN SPS hyperon beam. This value is 2.5 standard deviations higher than the current world average.     

The effect of sintering temperature on the intergranular properties of Bi2223 superconductors

A systematic study of the intergranular properties of (Bi,Pb)₂Sr₂Ca₂Cu₃O_y (Bi2223) polycrystalline samples has been done using the electrical resistivity and Ac susceptibility techniques. In this project, we have prepared a series of Bi2223 samples with different sintering temperature. The XRD results show that by increasing sintering temperature up to 865 °C the Bi2212 phase fraction decrease. It was found that the Bi2212 phase on the grain boundaries is likely to play the role of weak links and consequently reduces the intergranular critical current densities. Analysis of the temperature dependence of the Ac susceptibility near the transition temperature (T_c) has been done employing Bean's Critical State Model. The observed variation of intergranular critical current densities (J_c) with temperature indicates that the weak links are changed from superconductor–normal metal–superconductor (SNS) for well-coupled sample to superconductor–insulator–superconductor (SIS) type of junctions for the sample with high Bi2212 phase fraction.     

Pinning mechanism in carbon nanotube doping of MgB2 superconductor

MgB_1.9C_0.1 samples are synthesized under the ambient pressure (AP) and high pressure (HP), respectively. The further studies demonstrate different field-dependence of the critical current density J_c(H) in each sample. In the view of two-gap superconductivity in these samples, δT_c pinning (resulting from the spatial fluctuations of the transition temperature) is dominant in the AP sample, while in the HP sample, both δT_c and δl pinning (due to the mean-free-path fluctuations) act together and their contributions vary with temperature. Besides the improvement of H_c2(0), due to the different pinning mechanism, J_c(H) of the HP sample shows a slower decay with the increasing fields than that of the AP sample in high fields, which suggests a possible method of retarding the rapid decay of J_c(H) under elevated fields.     

Resonant paramagnetic enhancement of the thermal and zero-point Nyquist noise

The interaction between a very thin macroscopic solenoid, and a single magnetic particle precessing in a external magnetic field B₀, is described by taking into account the thermal and the zero-point fluctuations of stochastic electrodynamics. The inductor belongs to a RLC circuit without batteries and the random motion of the magnetic dipole generates in the solenoid a fluctuating current I_dip(t), and a fluctuating voltage ε_dip(t), with spectral distribution quite different from the Nyquist noise. We show that the mean square value 〈I_dip²〉 presents an enormous variation when the frequency of precession approaches the frequency of the circuit, but it is still much smaller than the Nyquist current in the circuit. However, we also show that 〈I_dip²〉 can reach measurable values if the inductor is interacting with a macroscopic sample of magnetic particles (atoms or nuclei) which are close enough to its coils.     

Implementation of the Astrov method for measuring the ME E effect with the use of a vibrating-coil magnetometer

A setup is developed for measuring the ME _E effect by fixing the amplitude of magnetization oscillations upon the repolarization of a sample, caused by the application of ac voltage to the sample plates. The temperature dependence of the ME _E effect in a Ga_{2 ? x} Fe _x O₃ single-crystal sample is measured in external fields from 0.25 to 1 kOe at temperatures from 77.4 to 280 K. It is established that the effect disappears when the Curie temperature is attained. The hysteresis of the magnetoelectric effect related to the magnetization hysteresis is measured.     

Relaxation modes in glassy polymers: A temporal analysis by the simplex method of isothermal depolarisation current measurements

Molecular dynamics associated to relaxation phenomena in the glass transition temperature domain is often investigated by means of thermostimulated depolarisation current TSDC technique. It is a very sensitive method and the data are traditionally obtained according to two protocols leading to the well known complex spectra and elementary spectra. The aim of this work is to use a new TSDC protocol which analyses the relaxation current kinetics obtained after submitting the sample to an electrical field pulse at a constant temperature. A new temporal analysis of the return equilibrium isothermal transient current I(T) is proposed. The signal fitting is obtained by a simplex optimisation method. The entire signal recorded for all the temperatures can be fitted with a sum of two exponentials allowing the definition of two different relaxation times called τ₁ and τ₂. This new protocol has been used to analyse the glass transition domain of amorphous PET.     

Current-induced effect on resistivity and magnetoresistance of La0.67Ba0.33MnO3 manganite

The influence of dc biasing current on temperature dependence of resistivity and low-field magnetoresistance (MR) of La_0.67Ba_0.33MnO₃ bulk sample is reported. A prominent finding is the change in resistivity around the insulator-to-metal transition temperature (T_IM) and the change in MR around the ferromagnetic transition temperature (T_C). The decrease in MR around T_C at higher biasing current indicates a strong interaction between carrier spin and spin of Mn ions resulting in a higher alignment of Mn ion spins. Change in resistivity around T_IM is interpreted in the framework of percolative conduction model based on the mixed phase of itinerant electrons and localized magnetic polarons.     

DC current effect on the magnetic phase and transport properties of polycrystalline La0.7Ca0.3MnO3

The influence of DC current on the resistivity and phase transition of polycrystalline La_0.7Ca_0.3MnO₃ has been investigated. The specific heat measurement found that charge carriers and ferromagnetic spin-wave contributions were changed after applied DC current. Applying high electric fields leads to the formation of ferromagnetic regions. The resistivity drops abruptly once the percolating current path is established. As current through the sample disappears, the larger ferromagnetic (FM) clusters, however, remain and are frozen in giving a measurable contribution to the specific heat of the system. The larger clusters should give rise to the value of spin-wave stiffness constant (D), as it is expected to increase the strength of the ferromagnetic coupling. The metallic ferromagnetic regions would make the charge carrier delocalization and attribute to specific heat linear term γ.     

The quantitative measurement of electromigration in CeO2−x

We present an experimental device for the measurement of electromigration in a non stoichiometric oxide which is composed principally of two symmetrical electrolytic cells: CeO_2?x-(ZrO₂, Y₂O₃)-CeO_2?x+? and which permits one to analyse the variation of the oxygen composition at the extremities of a CeO_2?x sample subjected to an electric field.The quantitative measurement of the effective charge and ionic transport numbers are discussed in terms of the reduction or elimination of such undesirable but inherent effects as the exchanges of oxygen between the sample and its neighbourhood and of the heat associated with the electrical current.     

Electrodynamic features of anisotropic hard superconductors

The low-frequency electromagnetic response of superconducting plates characterized by strong anisotropy of current-carrying capacity in the plane of the sample is studied experimentally and theoretically. Measurements are made on polycrystalline textured plates of the Y-123 system with the c axis lying in the plane of the sample and on a single crystal with a single preferred direction of twinning boundaries. It is shown that the shape of the curves describing the dependence of the relative losses q on the ac field amplitude h ₀ is quite sensitive to the orientation of vector h ₀ in the sample plane. As in the case of isotropic samples, the q(h ₀) dependence is characterized by a single size peak if vector h ₀ is oriented along one of the principal symmetry directions of the anisotropic critical current density. If h ₀ deviates considerably from principal directions, two size peaks are observed on the q(h ₀) curve. A detailed analysis of the evolution of the q(h ₀) curves upon a rotation of vector h ₀ in the sample plane is carried out.     

The influence of molecular hydrogen on the conductivity of magnesium oxide containing paramagnetic surface defects

MgO with a high concentration of defects was produced by the dehydration of high-purity Mg(OH)₂ and the conductivity was studied in the temperature range 20–250°C. An atmosphere of hydrogen increased the conductivity by up to a factor of 10³ and produced a characteristic conductivity maximum when the sample was heated. The hydrogen interacts with the sample at V¹_i-centers which are located at the surface of the dehydrated material. These centers are supposed to have a catalytic activity, i.e. they decompose hydrogen molecules into atoms and further into electrons and protons which contribute to the current.